It is well known in the art that certain harmonic components based on a fundamental frequency (i.e., operating frequency) of a radio frequency (RF) signal fed to an amplifier circuit can degrade linearity and efficiency characteristics of the amplifier. Proper treatment of the harmonic components can shape the waveform of the RF signal (e.g., at an output of the amplifier), so to enhance efficiency and other performance parameters of the amplifier. Teachings according to the prior art address such degradation by attenuating of the harmonic components via harmonic terminations (e.g., harmonic impedances such as harmonic shorts or harmonic open) coupled to an input and/or output of the amplifier. In such teachings, harmonic terminations that are tuned at harmonic frequencies of the fundamental frequency are used to attenuate amplitude of the harmonic components so to reduce their impact on a signal amplified by the amplifier.
As described in the above referenced U.S. Pat. No. 9,602,063 B2, the disclosure of which is incorporated herein by reference in its entirety, a second harmonic (i.e., second order product) can mix with the fundamental to generate third-order intermodulation products close to the fundamental frequency, and thus affect linearity. Attenuation of the second harmonic, via for example, a second harmonic short tuned at the second harmonic frequency, can therefore improve linearity. Such reference applies harmonic terminations at the output of the amplifier to attenuate second order and higher harmonic components and therefore maintain a desired linearity performance of the amplifier.
As described in the above referenced paper from Abbasian et al., the disclosure of which is incorporated herein by reference in its entirety, harmonic components at the output of the amplifier may couple to the input of the amplifier via nonlinear couplings (e.g. nonlinear transconductance, nonlinear capacitance) inherent to the amplifier design, and thereby affect linearity and efficiency characteristics of the amplifier. Abbasian et al. address existence of such nonlinear couplings by placing, at the input of the amplifier, harmonic terminations tuned at the harmonic frequencies of the fundamental frequency so to reduce amplitude of the coupled harmonics components at the input.
It would be clear to a person skilled in the art that both above references seek to maintain linearity and efficiency characteristics of the amplifier while considering effects of an input signal to the amplifier having a dual tone or complex modulated waveform, determined via, for example, well-known in the art two-tone IMD measurement techniques. Such techniques use two single frequency signals (tones) at different frequencies as input to the amplifier, to characterize corresponding intermodulation products (i.e. harmonic components) generated at input and/or output of the amplifier (in response to the two tones) that are indicative of linear distortion. Such characterization can serve as basis for design of harmonic terminations tuned at the harmonic frequencies for reducing of the intermodulation products, thereby improving linearity.
However, current modulation schemes may include single tone RF signals, where a down converted harmonic component from the second harmonic to the fundamental frequency (i.e., frequency of the single tone) may affect amplitude and phase at the output of the amplifier. Such effect on the amplitude and phase at the output of the amplifier may be measured in terms of well-known in the art metrics such as AM-AM distortion and AM-PM distortion. Attenuation of harmonic components via harmonic terminations tuned at harmonic frequencies of the fundamental frequency, as taught by the above discussed references, may not be sufficient in providing an increased performance in terms of such metrics for a case of a single tone RF signal. Teachings according to the present disclosure are aimed to reduce AM-AM distortion and AM-PM distortion while, in some cases, sacrificing a performance in the attenuation of the harmonic components.